1. Field of the Invention
The present invention relates to (i) an X-ray optical apparatus preferably used for an X-ray exposure system for fabricating a semiconductor device and (ii) an X-ray microscope system.
2. Description of the Related Art
The X-ray exposure method is one of the methods for fabricating a micro device, such as a semiconductor circuit device, having a fine pattern. In the case of this method, a mask having a circuit pattern formed thereon is illuminated with X-rays, the image of the mask is reduction-projected onto the surface of a wafer, and a resist on the wafer surface is exposed to transfer the circuit pattern from the mask.
FIG. 11 is a schematic block diagram of a conventional X-ray exposure system. In FIG. 11, reference numeral 901 denotes an undulator light source for emitting a light beam such as an X-ray beam, 903 denotes an X-ray beam emitted by the light source 901, 904 denotes a total reflection mirror, 909 denotes a reflection-type mask, 910 denotes a mask stage, 911 denotes a projection optical system, 912 denotes a wafer, 913 denotes a wafer stage, and 916 denotes a vacuum vessel for containing the aforementioned elements in a vacuum ambience.
This exposure system comprises the undulator light source 901 serving as an X-ray source, the total reflection mirror 904 serving as an illumination optical system, the reflection-type mask 909, the stage 910 with the reflection-type mask 909 mounted on it, the projection optical system 911, the wafer 912, the stage 913 with the wafer 912 mounted on it, an alignment mechanism for accurately and relatively adjusting the position of the mask 909 and the wafer 912, the vacuum vessel 916 for keeping the entire exposure system in a vacuum state, in order to prevent X-rays from attenuating, and an exhauster (not shown).
The undulator light source 901 constitutes an X-ray source. The undulator light source 901 emits so-called pencil-beam-like X-rays 903 which are thin and parallel. The X-rays 903 are reflected by the total reflection mirror 904 to cut short-wavelength components and form the X-rays into an almost-monochromatic X-ray beam 905, and apply the beam 905 to the reflection-type mask 909. The light emitting point of the undulator light source 901 is close to a point light source, because it has a size of approximately a few hundreds of microns. Therefore, the spread of the angle of the X-rays applied to one point on the mask 909 is very small.
The reflection-type mask 909 is provided with an X-ray reflection film pattern corresponding to a circuit pattern to be transferred and X-rays incident on the mask are reflected and led to the projection optical system 911. Then, the image of the circuit pattern of the reflection-type mask 909 is reduction-projected onto the surface of the wafer 912 to expose the resist. The projection optical system 911 comprises a plurality of multilayer-film reflection mirrors to reduction-project a pattern on the mask 909 onto the surface of the wafer 912. The projection optical system 911 normally uses a telecentric system.
However, an illumination optical apparatus used for a conventional X-ray exposure system has the following drawbacks, which need to be improved. That is, when a very fine pattern on a mask is projected onto a wafer, the resolution and the focal depth are insufficient and therefore, it is impossible to transfer a pattern at an accuracy necessary for the fabrication of a next-generation device. Moreover, even when a phase shift mask is used, it is impossible to adequately obtain the desired imaging performance, which should be improved by using the phase shift mask. The reason for this is described below.
A coherence factor a is one of the parameters for demonstrating the characteristics of an illumination system. When it is assumed that the numerical aperture at the mask side of a projection optical system is NApl and the numerical aperture at the mask side of an illumination optical system is NAi, the coherence factor is defined by the following expression : EQU .sigma.=NAi/NApi.
The optimum value of the coherence factor a is determined by necessary resolution and contrast. In general, an interference pattern appears at the edge of the image of a fine pattern projected onto a wafer when the coherence factor .sigma. is too small, but the contrast of a projected image lowers when the coherence factor .sigma. is too large.
The case of the coherence factor .sigma. being 0 is referred to as coherent illumination. In this case, the optical transfer function (OTF) of an optical system shows a relatively large constant value up to a spatial frequency given by NAp2/.lambda. by assuming the numerical aperture at the wafer side of a projection optical system to be NAp2 and the X-ray wavelength of the optical system to be .lambda.. However, the OTF becomes 0 for a high frequency exceeding the spatial frequency and therefore, the fine pattern cannot be resolved. The case of .sigma. being 1 is referred to as incoherent illumination. In this case, the OTF decreases as the spatial frequency increases, but it does not become 0 up to a spatial frequency given by 2.times.NAp2/.lambda.. Therefore, it is possible to resolve up to a finer pattern. To transfer a pattern having a structure larger than a diffraction limit, it is preferable that the coherence factor a has a value closer to that providing coherent illumination. Moreover, also when a phase shift mask is used, the effect of the improved imaging performance may adequately be obtained when the coherence factor .sigma. has a value closer to that providing coherent illumination. However, to transfer a pattern having a fine structure close to the diffraction limit, it is preferable that the coherence factor .sigma. has a value closer to that providing incoherent illumination, from the viewpoint of contrast.
In the case of the conventional example discussed above, the coherence factor .sigma. has a value almost equal to the condition of the coherent illumination because the spread of the angle of X-rays for illuminating one point on a mask is very small and the value of the coherence factor .sigma. is close to 0. Therefore, there is a disadvantage that a fine pattern cannot be transferred. The present invention is made to solve at least this problem.